Postponed on:
#3050374: Create Drupal 9 stable theme
#3110855: Plan for removing dependency to Stable in Bartik/Seven/Claro/Umami
#3115088: Remove Classy as a base theme of core themes
Problem/Motivation
Stable will be moved to contrib during Drupal 9. After we have ensured that core themes (Bartik, Claro, Seven, Umami) are not using Stable templates or libraries, we can remove Stable as a base theme of these themes.
Proposed resolution
Set base theme: false for Bartik, Claro, Seven, and Umami
Remaining tasks
Confirm all @todo's referencing this issue are addressed.